In the transmission of binary coded signals via a power interface, line characteristics, in particular, line inductance and sensor capacitance, but also the load capacitance of other sensors (transmitters) arranged in parallel in a bus system, have a significant influence on the distortion of the signals coming into the receiver. Such distortion makes evaluation on the receiver side more difficult or even impossible under some circumstances.
Linear distortion is caused, in particular, by capacitance on the sensor side. Together with the line inductance, it forms an oscillating circuit that can cause strong linear distortion of the binary signals to be transmitted.
The capacitance on the sensor side of a power interface suppresses the coupled noise voltage and is thus unavoidable. This means that the different spectral components of the transmission signal have linear distortion in amplitude and phase on the transmission line. This has the result that, for example, a transmitted, approximately square-wave signal reaches the receiver side with more or less distortion. This makes the error-free reception of the binary signal significantly more difficult on the receiver side.
To avoid such distorting line characteristics, the transmission lines are typically “terminated” on both ends with the characteristic impedance, e.g., with 120 ohms at each end of a double circuit line.
In principle, this is also possible in so-called power interfaces. However, because the power for powering the sensor must also flow via the signal lines, terminating resistors on this order of magnitude are undesired. They lead to an increased voltage drop and an increased power loss. This is because, in an example calculation where the supply voltage on the terminals of one sensor is 5 V, the resistance is 120 ohm, and the current is 20 mA, only a voltage of 5.0 V−2.4 V=2.6 V remains for the sensor circuit. To avoid this voltage drop or to keep it small, only an attenuation resistance of a few ohms, instead of 120 ohms, can be accepted. For this reason, the design must use non-nominally tuned termination resistors. Increasing line length and the resulting increasing line inductance causes ringing or attenuated oscillation at each signal edge.
From DE 10 2004 030 728 A1, a method for reducing noise in signal transmission is known, especially in pulse-width-modulated signals. Here, for changing the edge profile when an edge to be changed is applied to an input of the device, a digitized edge profile stored in the memory is read out from the memory and converted by means of the D/A converter into an analog signal with the changed edge profile.
The task of the present invention is therefore to disclose a method for reducing linear distortion in a power interface for a motor vehicle which automatically produces an absolute minimum of the harmonics contained in the signal.